Neurodegenerative diseases are progressive diseases in which neuronal cells in the central nervous system gradually degenerate and die thereby. Among neurodegenerative diseases, it is known that abnormal accumulation of α (alpha)-synuclein occurs in synucleinopathies including Parkinson disease (PD), dementia with Lewy bodies (DLB), multiple system atrophy and the like. Further, it is known that abnormal accumulation of amyloid-β (beta)-protein and tau occurs in Alzheimer disease (AD). Furthermore, it is also known that abnormal accumulation of tau occurs in progressive supranuclear palsy, corticobasal degeneration and Pick's disease. Here, it has been elucidated that since genetic abnormalities in these proteins also cause familial neurodegenerative diseases, abnormal aggregation/accumulation of these proteins in the neuronal cells of the central nervous system leads to acquisition of neurotoxicity, i.e., mechanisms causing neuronal cell death or the like have mainly resulted in pathologies of neurodegenerative diseases.
Further, neurodegenerative diseases including Alzheimer disease and tau disease are caused by phosphorylated tau. Tau protein plays a role as a hinge between tubulins by binding to microtubules serving as structural proteins of neuronal cells, and has an ability to promote polymerization of tubulins into microtubules. When phosphorylated, tau protein loses the microtubule-binding ability, thus causing a collapse of normal cytoskeleton. Further, the phosphorylated tau protein causes neurodegeneration involving neurofibrillary tangle, accompanied with forming Paired Helical filaments (PHF). With regard to Alzheimer disease (AD), abnormal accumulation of amyloid-β (beta)-protein and tau is mainly observed. Further, accumulation of α (alpha)-synuclein is also observed in the brains of a large number of AD patients. Particularly, with regard to the ceruleus nuclei and amygdalae of the AD patients, it is known that Lewy bodies composed of α (alpha)-synuclein or the like and neurofibrillary tangle caused by tau protein colocalize in an identical neuronal cell. Further, phosphorylated tau and α (alpha)-synuclein exist in an identical location of a halo section of a brainstem-type Lewy body, and are assumed to together promote formation of neurofibrillary tangle. In fact, since AD is likely to be complicated by PD, and PD is likely to be complicated by AD, it is assumed that AD and PD are associated with each other at the molecular level.